1. Field of the Invention
The present invention relates to furnaces used generally for some form of chemical processing and particularly to such furnaces that provide a controlled variation in temperature over the processing time.
2. Description of the Prior Art
A particular application of the present type of furnace is in the growing of single crystals from a "melt". One common way to grow single crystals from a melt is to place the crystalline material in a shaped refractory container and then heat it in a furnace until it is molten. The molten material is then cooled slowly at one end until crystallization sets in starting from a single nucleation. Provided the cooling to crystallization temperature is done slowly and progressively along the length of the material, the crystal lattice can form entirely on the single nucleation so as to produce a single crystal. The usual optimum conditions for single crystal growth call for a rather steep temperature gradient that moves relative to the material. The two most common ways of moving the temperature gradient relative to the material are: (a) to physically move the material inside the furnace and (b) to hold the material stationary while moving the furnace. In the above two systems, the temperature zones inside the furnace are maintained constant.
A third way is to move the isotherm inside the furnace by means of temperature controls. This third way has not been popular since the approaches used have not provided a sufficiently precise moving gradient. The first two ways require the complication of physical transport with attendant heat loss and physical size problems.